"There was a reduction in monitoring activities related to cameras and monitoring systems," Rafael Grossi told reporters at Vienna airport after returning from Tehran. "We have agreed that both will be operating again."

The agreement was announced after the UN nuclear watchdog chief said Saturday concluded what he called "constructive" meetings with Iranian officials in Tehran after the discovery of uranium particles enriched to near weapons-grade level.

The two-day visit by the head of the International Atomic Energy Agency (IAEA) comes as the Vienna-based organisation seeks greater cooperation with Iran over its nuclear activities.

"By having a constructive discussion... and having good agreements, like I am sure we are going to have, we are going to be paving the way for important agreements," Rafael Grossi told a news conference alongside Iran's top nuclear official Mohammad Eslami.

Grossi arrived in Iran on Friday amid deadlock in negotiations on reviving a landmark 2015 accord on Iran's nuclear activity, known formally as the Joint Comprehensive Plan of Action, or JCPOA.

Grossi met later on Saturday with Iranian Foreign Minister Hossein Amir-Abdollahian, official news agency IRNA reported.

Iran's top diplomat earlier told CNN that "the window for an agreement on negotiations to lift sanctions is still open, but this window will not be open forever".

The visit comes after Uranium particles enriched up to 83.7 percent -- just under the 90 percent needed to produce an atomic bomb -- had been detected at Iran's underground Fordow plant about 100 kilometres (60 miles) south of the capital, according to a confidential IAEA report seen by AFP this week.

The discovery came after Iran had substantially modified an interconnection between two centrifuge clusters enriching uranium, without declaring it to the IAEA.

Tehran denies wanting to acquire atomic weapons, and said it had not made any attempt to enrich uranium beyond 60-percent purity.